The present invention relates generally to the field of noise cancellation and more particularly to a system and method for noise cancellation in apparatus such as electric motors and generators.
Electric motors and generators generate a substantial amount of tonal noise during operation. In air-cooled generators, for example, the excitation of the tonal noise comes from two major sources: the electromagnetic force and the rotor jets. The noise from the air jets is typically the main source of tonal noise. The air jets create a tonal noise at a fundamental frequency that equals twice the rotational frequency of the rotor. For example, in a two-pole 60 Hz power generator, the fundamental tonal noise has a frequency of 120 Hz. Because of its particular frequency and amplitude, the fundamental tonal noise may be especially annoying to human ear perception.
To meet customer specifications or regulatory requirements, a number of solutions exist to reduce the repetitive noise produced by an electric motor or generator. One approach is to build acoustic walls around the motor or generator so that it becomes isolated in a sound-proof housing. However, this solution is usually expensive and may not always be feasible to practice. Another approach is to attach acoustic panels inside the stator frame or to cover the outside of the motor or generator with acoustic blankets. Due to the fact that the motor or generator surface cannot be fully covered, the noise reduction effect is not as good as that from the surrounding walls.
Other noise reduction solutions focus on active noise cancellation, which involves actively detecting the amplitude, frequency and phase of each of the component waves of the noise in real time, and through complex feedback looped circuitry, generating waves or vibrations of similar amplitudes, frequencies and 180-degree different phase angles (opposite phases), to cancel out the effect of the noise waves or vibrations. However, the active noise cancellation approach usually involves a complicated setup of input sensors, feedback loop logic, and output acoustic sources. Thus, active noise cancellation is usually expensive to implement.